Transmitting system



July 4, 1 P. B-ARTo-N Y 'TRANSMITTINGQYSTEM Filed' June-is, 11925 a? sa 38 37 LIllllflllg j l ATTORNEY Patented July 4, 1933 Y UNITED STATES PATENT OFFICE JAMES 1. BARTON, OE- SPRINGFIELD, MASSACHUSETTS, ASSIGNOR TO WESTINGHOUSE .ELECTRIC & MANUFACTURING COMPANY, A. CORPORATION OF PENNSYLVANIA TRANSMITTING SYSTEM Application ala .Tune 18,

My invention relates to transmitting systems, either radio or Wire, and it has particular relation to means for deriving telegraphic signal-impulses previous to their utilization for modulating the output of such systems.

Commercial high-power radio-transmitters, at the present time, are usually situated at considerable distances from the points at which the telegraphic signals, to be transmitted, originate. The transmitters, are, preferably, remotely controlled from a central station and the signals are conveyed thereto by carrier currents, interrupted at signalf intervals, over metallic conductors.

During the process of keying thecarrier-current generators at the central station, undesirable transient frequencies are introduced into the signal-impulses at the beginning and end of each impulse. Such frequencies may be caused by sparking at the key-terminals, shock-excitation of the various circuits, such as filter circuits, the characteristics of the metallic lines between the central station and the transmitter and by one or more of a great many other conditions. Other undesirable voltages which cause line noise may also be introduced into the signal impulse unless special precautions are taken.

Irrespective of the cause of the line-noise voltages or of the start-and-stop transients,

as they are known to those skilled in the art, their presence in the signal, when received at the transmitter, is objectionable, since, when the transmitter is modulated thereby, the transmitted signals are not clean and cannot be satisfactorily received by automatic recorders.

Itis, accordingly, an object of my invention to provide means whereby transient and line-noise frequencies, accompanying signal impulses, are prevented from being impressed on a radio transmitter.

Another object of my invention is to provide a signal-impulse shaping network that shall eliminate the above-mentioned frequencies.

Another and more specific object 'of my invention is tf provide a signal-impulse shaping network that shall be capable of selecting 1929. Serial N0. 371,782.

a desired portion of an impulse impressed thereon, to the exclusion of start-and-stop transient frequencies.

In practicing my invention; after the signal impulses have been transmitted over the metallic line, they are rectified and then transmitted to the radio transmitter through a resistance-coupled amplilier comprising two adjacent stages of amplification which include thermionic amplifiers that have their control electrodes so negatively biased that a predetermined positive potential must be impressed on each control electrode before any current flows in the corresponding plate circuit. The signal pulse in its nal form decreases the negative bias of a modulator tube to permit the transmission of a carrier wave for the duration of the signal pulse.

Since the rectified signal impulses are pulses of direct current, the amplifier is designed to transmit direct current. This design includes so placing a battery in the grid circuit of each amplifier tube that the grid does not assume the positive potential of the plate of the preceding tube.

The operation of the resistance-coupled amplifier, as a current-limiting or signalshaping network, depends upon the fact that, in a resistance-coupled amplifier, the plate current of one amplifier tube is a4 minimum when the plate current of a preceding amplifier tube is a maximum. Because of this, characteristic of a resistance-coupled amplifier, and because the grids of two of the amplifiertubes are biased to a Adefinite cut-off point, one amplifier tube will determine the minimum `amplitude of the current transmitted, While a preceding tube will determine the maximum value of the vcurrent transmitted.

Other features and advantages of my invention will appear from'the following descript/ion, taken in connection with the accompanying drawing, in which;

Figure 1 is a schematic diagram of the apparatus and electrical connections employed in one embodiment of my invention.

Fig. 2 is a view of a curve "which indicates the character of a signal impulse, the shaded portion being that part of the impulse which is transmitted by the signal-shaping network.

Fig. 3 is a view of a curvewhch indicates the character of a signal impulse after it has passed through the signal-shaping network.

Referring to Fig. 1, the apparatus comprises a carrier-current telegraph transmitter 11 which is electrically connected to a transmission line 12 by means of either a capacity-coupling device 13 or an antenna. The carrier-current telegraph transmitter 11 comprises any well known means for transmitting a carrier current (of either audio o1' radio frequency), in the form of signal impulses, over the transmission line 12.

A radio transmitter 14 is connected to the transmission line 12, by means of an amplifier 15, a rectifier 16, a low-pass filter 17, and a signal-shaping network 18 constructed in accordance with my invention.

The amplier 15, which may be of any well known type, is connected to the transmission line 12 either by a capacity-coupling device 1 19 or by means of an antenna. The output of amplifier 15 is connected to the input of the full-wave rectifier 16 by means of a transformer 10.

The rectifier 16 comprises two rectifier tubes 2O and 21 of the hot-filament type, the filaments 22 and 23 of the tubes being connected together and grounded. The plate or anode 24 of the rectifier tube 20 is connected tothe upper end of the secondary winding 25 of the transformer 10, while the anode 26 of the rectifier tube 21 is connected to the lower end of the secondary winding 25.

The mid-point of the secondary winding 25 is connected to one input terminal of the low-pass filter 17, the filaments 22 and 23 of the rectifier tubes being connected to the other input terminal of the filter.

Thefilter 17 is of the well known low-pass type comprising series inductances 27 and shunt capacities 28 and is employed for 4suppressing carrier and associated frequenciesthat may appear in the output circuit of the rectifier 16, so that only the direct-current component will be passed by the filter.

The output terminals 1 and 2 of the filter 17 are connected to the input terminals 1 and 2 of the signal-shaping network 18 which includes the three-electrode thermionic amplifier tubes T1 and T2 and the associated elements which connect tube T1 to the filter 17 and which connect the tube T2 to the output of the preceding tube T1. The ampli-fier tube T3, which is of the usual three-electrode thermionic type, is merely a portion of a stage of resistance-coupled amplificationl for connecting the output of the signal-shaping network 18 to the input of the modulator tube 29. f

The filaments of the rectifier tubes 20 and'4 21, the amplifier tubes T1, T2. T1, and the modulator tube 29 are connected in parallel and fresistor R0 is connected to the conductor 31.

The grid 32 of tube T1 is connected to the positive terminal of a biasing battery 33, the other terminal of the biasing battery 33 being connected to the upper portion of the resistor R0 by means of an adjustable contact point 34. The filament 35 of tube T1 is connected to the conductor 31. The plate 36 of the tube lT1 is connected to the upper terminal of a Iresistor R1, the lower end of the resistor R1 bein connected to the positive terminal of the battery 30.

The grid 37 of tube T2 is connected to the negative terminal of a biasing battery 38, the positive terminal of the biasing battery 38 being connected to the upper portion of the resistor R1 by means of an adjustable contact point 39. The filament 40 of tube T2 is connected to the conductor 31. The plate 41 of tube T2 is connected to the upper terminal of the resistor R2, the lower terminal of which .is connected to the positive terminal of the B battery 30.

In a similar manner, the grid 42 of tube T 3 is connected to the negative terminal of the biasing battery 43, the positive terminal of which is connected to the resistor R2 by means of an adjustable contact point 44. The filament 45 of tube T1L is connected to the conductor 31. The plate 46 of tube T 3 is connected to the upper terminal ofthe resistor R3, the lower terminal of which is connected to the positive terminal of the B battery 30.

The modulator tube 29 is of a well known type comprising a three-electrode thermionic tu e. The grid 47 of the modulator tube 29 is connected to one terminal of the secondary 48 of a radio-frequency transformer 49. The other terminal of the secondary 48 is connected to the negative terminal of a biasing battery 50 through a radio-frequency choke coil or reactor 51. The positive terminal of the biasing battery 50 is connected to the upper portion of the resistor R3 by means of an adjustable contact point. The filament/52 of the modulator tube 29 is connected to the conductor 31. A condenser 53 is connected from a point between the transformer secondary 48 and the reactor 51 to the conductor 31 in order to complete a circuit for the radio-frey quency current which is supplied to the transformer secondary 48 through the primary winding 54.

The output circuit of the modulator tube 29 'nsl l voltage of the biasing battery 33 is such that,

is connected to the input of a radio-frequency power amplifier which .amplifies the radio-frequency carrier to the proper value before it is impressed upon the radiating eircuit 56.

It will be noted that the grid 32 of tube T1 is so biased that, when no current is flowing in resistor R0, the current in resistor R1 will be a maximum. When current flows in resistor R0, the voltage drop thereinopposes the voltage of the biasing battery 33, and the bias on the grid 32 becomes less positive as the current in resistor R0 increases. The

when the current in resistor R0 reaches a predetermined value, the bias of the grid 32 will ybe such that current will cease to flow in the resistor R1, that is, a cut-ol point will be reached. Therefore, a further increase of current in resistor R0 will not affect the value of the current flowing in resistor R1.

The voltage of the biasing battery 38, which is connected to the grid 37 of tube T2, is smaller than the voltage of the B battery 30. Therefore, when no current is flowing in the resistor R1, a`positive potential is impressed on the grid of tube T2. When current is flowing in the resistor R1, the voltage drop therein is in such direction as to aid the voltage of the biasing battery 38. Consequently, as the flow of current in resistor R1 increases,'the grid 37 of tube T2 becomes less positive. The voltage of the biasing battery 38 is of such value that, when the current in resistor R1 increases to a predetermined value, the bias of the grid 37 is such as to reduce the flow of current in resistor R2 to zero. Therefore, a further increase of current in resistor R1 will not affect the value of the current flowing in resistor R2.

The bias on the grid 42 of the tube T3 may be adjusted the same as the bias on the grid 37 of the preceding tube. This, however, is

' tery 50 is of such value that, when a current not essential, since the circuit will operate i properly if the current in resistor R3 never becomes zero.

The voltage of the modulator biasmg batof. a predetermined value is flowing in the resistor R2, the grid 47 of the modulator tube 29 is so biased that the output current of the modulator tube 29 is zero, although the radiofrequency carrier is being continuously impressed on themodulator input.

In operation, the carrier-current telegraph transmitter 11 transmits a carrier current, broken into signal-impulses, over the transmission line 12. The carrier current passes from the transmission line 12 through the coupling device 19 to the input of the amplifier 15. After being amplified, the impulses of lcarrier current are impressed on the input of the rectifier 16 by means of the transformer 10. The signal-impulses then appear in the output of the rectifier 16v as directcurIrent impulses of the character illustrated in i .2.

It vill be noted that there are transient currents superimposed on the direct-current pulses at the maximum amplitude of the pulse, and that there are also transient currents and line-noise currents at both the beginning and the. end of the pulse. The object of the circuit which is located between the filter 17 and the radio transmitter 14 is to eliminate these transient and line-noise currents and transmit only the current lying between the values T2 cut-off and T1 cutoff, as indicated by the shaded area in Fig. 2. We shall now consider the operation of the signal-shaping network 18 which includes the amplifier tubes T1 and T2 and their input circuits. During the period that no signal is being received, the' only current flowing through resistor R0 is that due to line noises and stray interference, and the current fiowin in said resistor is a minimum. There ore, maximum current is flowing in resistor R1, the grid 37 of tube T2 is biased beyond the cut-off point and no current is flowing in resistor R2.

As soon as a signal is transmitted from the telegraph transmitter 11, the signal voltage or signal impulse begins to build up, and a current, which is increasing in value, flows through resistor R0. The increase in current in RD decreases the positive potential on the grid 32 of tube T1, and the current in resistor R1C'clec1eases, thus decreasing the voltage drop in resistor R1 and mak ing the grid 37 of tube T2 more positive.

VVhenthe signal voltage builds up to a predetermined value, the voltage impressed on the grid of tube T2 reaches a value (represented by T2 cut-off in Fig. 2) above the cut-ofi po'int, and current flows in resistor R2. It will be noted that, before the signal voltage reaches this predetermined value, the signal impulse transients and the voltage variations representing line noise do not affect the current in resistor R2 and, therefore, do not affect the modulator output.

As the signal voltage further increases in value, the current in resistor R2 increases, thi current in resistor R1 decreases and the cur rent in resistor R2 increases. Vhen the signal voltage builds up to a second predetermined value, the current in resistor R0 is of such magnitude (represented by T1 cutoff in Fig. 2) that the grid 32 of tube T1 is biased to the cut-off point, and the current in resistor R1 becomes zero. The current in resistor R2 is nowofmaximum value since further increase in signal voltage does not affect the current in resistor R1, and said further increase does not afl'ect the current in resistor R2.

From the abovedescription, it is apparent that there is no current flowing in resistor R2 until the signal voltage reaches a predetermined value, and that, when the signal voltage further increases to a second predetermined value, the current in resistor R2 reaches a maximum value which is not af- -fected by the signal voltage increasing beyond said second predetermined value. Accordingly, the line-noise voltages and transients indicated in Fig. 2 are suppressed, and only the portion of the signal impulse which\is represented by the shaded portion is transmitted by the signal-shaping net- Work 18.

The current flowing in resistor R2 controls the output ofmodulator tube -29 as follows: When the current in resistor R2 increases, the current in resistor R3 decreases, thereby so decreasing the negative bias on the grida? of the modulator tube 29 as to permit the radio-frequency carrier to appear in the modulator output and to be amplified and radiated.

The signal impulse which is finally impressed upon the input of the modulator tube 29 is of the character indicated in Fig. 3.

Various modifications may be made in my invention Without departing from the spiritand scope thereof, and I desire, therefore, that only such limitations shall be placed thereon as are shown b the prior art and set forth by the appen ed claims.

I claim as my invention:

1. Electrical apparatus comprising means for transmitting electrical energy only when said energy reaches a predetermined magnitude, a second means for transmitting electrical energy only when said energy reaches a predetermined magnitude and means for so electrically connecting said first means and said second means that, when the electrical output of one of said means is a maxi-` mum, the electrical output of the other of said means is a minimum.

2. Electrical apparatus comprising an amplifier having a control electrode, means for making said amplifier responsive only to voltages above a predetermined value, a second amplifier having a control electrode, means for so coupling said amplifiers that, when the control electrode of said first amplifier becomes more positive, vthe control electrode of said second amplifier becomes less positive and means for making said second amplifier responsive only to voltages above a predetermined value.

3. Electrical apparatus comprising a thermionic amplifier having a control electrode, means for so negatively biasing said electrode that said amplifier is inoperative until the voltage impressed on the input of said amplifier reaches a predetermined value, a second thermionic amplifier having a control electrode, means forv so negatively biasing the control electrode of said second amplifier that said second amplifier is inoperative until the voltage impressed on the input of said second amplifier reaches a predetermined value and means for so electrically coupling said amplifiers that, when the output of one amplifier is a maximum, the output of the other amplifier is a minimum.

` 4. Electrical apparatus comprising a thermionic amplifier having a control electrode, means for so biasing said electrode that said amplifier is inoperative until the voltage impressed on the input of said amplifier reaches a predetermined value, a second thermionic amplifier having a control electrode, means for so biasing the control electrode of said second amplifier that said second amplifier is inoperative until the voltage impressed on the input of said second amplifier reaches a predetermined value and means, including a resistance unit common to both the output of said first amplifier and the input of said second amplifier, for so electrically cou ling said amplifier that, when the output olf one amplifier is a maximum, the output of the other amplifier is a minimum.

5. Electrical apparatus comprising a ther-l mionic amplifier having a control electrode, means for so biasing said electrode that said amplifier is inoperative until the voltage impressed on the input of said amplifier reaches a predeterminedvalue, a second thermionic amplifier having a control electrode, means for so biasing the control electrode of said second amplifier that said second amplifier is inoperative until the voltage impressed on the input of said second amplifier reaches a predetermined value and means, including a resistance unit common to both the output of said first amplifier and the input of said. second amplifier, for so electrically coupling said amplifiers that direct current is amplified by said electrical apparatus.

6. Electrical transmitting apparatus comprising means for transmitting' pulses of alternating current over a transmission line, means for rectifying said'current after it has been transmitted over said line, transmitting means connected to said rectifier for transmitting electrical energy only when said current reaches a predetermined value, a second transmitting means for transmitting electrical energy only when said energy reaches a predetermined value, means for so electrically connecting said first transmittingmeans to said second transmitting means that, when the electrical output of one of said transmitting means is a maximum, the output of the other of said transmitting means is a minimum, a radio transmitter, means responsive to pulses of current for modulating the output of said radio transmitter and means for electrically connecting said second-named transmitting means and said last-named means.

7. A signalling system comprising means for transmitting a carrier current in the form of abruptly interrupted pulses of current,

Lit

means for receiving and rectifying said pulses of current, transmitting apparatus including a modulator responsive to said rectified pulses of current, and means free of mechanical inertia for transmitting said rectified pulses to said modulator and for substantially eliminating transients from said pulses.

8. The method of keying a radio transmitter which comprises transmitting a signal impulse in the form of an alternating current, rectifying said signal impulse, so modifying said signal impulse that its amplitude is be tween two limiting Values, and impressi" said modified impulse on the inputoffsaid transmitter.

9. The method of changing the bias of a modulator tube by a predetermined constant amount for a short interval which comprises transmitting a signal impulse in the form of an alternating current, rectifying said signal impulse, so modifying said signal impulse direct current, said pulse having values above one limit and below another limit, eans for so modifying said pulse that its amplitude is ,betweellf-said two limits, and means for im- .*pressin'g said-'Imodiiied pulse on the input of -said transmitter.

In testimony whereof, I have hereunto subscribed my name this 11th day of June 1929.

JAMES P. BARTON. 

